The film opens in a forest in Ireland. A man named John (Alistair Brammer) is running frantically but seems to find himself coming back to the same place or past multiple signs that say “point of no return” with different numbers on them. An unseen force then violently drags John away.
Elsewhere in the country is an artist named Mina (Dakota Fanning). She lives alone and has dealt with the guilt over her mother’s death in a car accident 15 years earlier because Mina was fooling around and mocking her mother when she told her to stop. Mina works at a pet store and is asked by her boss to deliver a parakeet that she later names Darwin. Before her trip to deliver the bird, Mina goes out wearing a wig and using a fake name, which she tells Darwin is because she prefers to pretend to be someone else.
Mina takes the trip with Darwin and ends up going down a road leading into the forest. The car suddenly breaks down and shuts off, leaving Mina stranded. She walks out of the car with Darwin and calls for help, but when she comes back to the spot where she left the car, it is gone. Mina continues to wander until she sees a woman (Olwen Fouere) running, and Mina chases after her. As it is getting darker, the woman enters a small cabin and tells Mina to go inside before it’s too late. Mina runs into the cabin.
The old woman says her name is Madeline, and she is with two other young people, Ciara (Georgina Campbell) and Daniel (Oliver Finnegan). The cabin they are in is called “The Coop”, and they have all been there for an undisclosed amount of time. Mina stands by the mirror and hears what sounds like applause. The sound is coming from the outside, as the four are being observed by entities known only as “The Watchers”.
Daniel puts on a show of sorts for The Watchers, as Mina is told to just act normal while they are being seen. The three give Mina rules for staying in the Coop, as well as for surviving The Watchers. They are able to leave the cabin during the daytime since The Watchers cannot come out in the sunlight, and they must be indoors when it gets dark. Mina bonds with Ciara, who says she came with John, who was her husband, and it is suggested that Daniel has a crush on Ciara.
Mina convinces Daniel to help her go near the hole where The Watchers dwell during the daytime. Using a rope to descend, Mina comes down and finds several artifacts, including a picture of a man that Madeline mentioned called “The Professor” (John Lynch). A Watcher then appears to approach Mina, making horrifying noises before stretching out a hand with six fingers. Daniel throws the rope back down to pull Mina back up.
Back in the Coop, someone appearing to be John comes knocking at the door. The man can only be seen from his feet through surveillance cameras. Ciara really believes it is him, but Madeline forbids her from opening the door, as it could be a Watcher mimicking John to make their way in. Ciara tries to prove it’s John by asking him what book she’s reading, but “John” doesn’t answer and is seemingly killed by a Watcher.
After an argument, Mina and Madeline get left outside before night falls, and Ciara and Daniel bar the door to prevent The Watchers from coming in. The two ladies hide as The Watchers emerge, appearing as tall, lanky, spider-like monsters that let out horrible screeching sounds. They stand by the window and watch Daniel while Mina and Madeline wait until The Watchers leave so they can get back into the Coop.
The four discover a basement beneath the Coop that they never knew was there. Upon entering, they find several artifacts belonging to The Professor, whose real name was Rory Kilmartin. When he was alive, Kilmartin studied The Watchers and tried to learn of their origins. Mina finds videos on a computer left by Kilmartin where he documented his time with a captured Watcher, trying to convince it that he was harmless and wanted to learn from it. The last video shows that Kilmartin eventually came to realize helping The Watchers was futile, so he used a gun to kill the captive Watcher and himself.
Madeline then tells the others what the Watchers really are – they are fairies that once inhabited the world of man before being banished. The one time she got close enough to see a Watcher, she saw that it looked sort of like her, but more of an imperfect copy. The group then makes an escape plan to get on a boat once they go past the point of no return. They get to the boat as night falls, but Daniel gets killed by The Watchers, who then take on his form, while Mina, Madeline, and Ciara manage to get away.
The women wake up as the boat makes it back to civilization. After settling back to normal, Mina goes to the university where Kilmartin worked and poses as his niece to get into his office to fulfill his last request. There, Mina finds more documents, pictures, and a recording from Kilmartin discussing halflings and how The Watchers could have been used to bring people back. Mina sees one set of pictures, and she appears surprised.
Mina goes to Ciara’s house to tell her that she believes that the Madeline they met was a Watcher the whole time, specifically a halfling. The pictures Mina found showed that Kilmartin was married to Madeline, but she died in 2001, and the halfling took her form. It was also the same Watcher that Kilmartin had captured, as he hadn’t killed it and it had assumed Madeline’s form to return to civilization since it can walk in the daylight. Outside, Mina sees the real Ciara returning home, as the one she was talking to is the Daywalker. When Ciara arrives, the Daywalker knocks her out before going after Mina as Madeline. Mina talks the Daywalker down and tries to convince it that there are others like it out there, and admits it was not fair of them to be cast out from their world. She also tells the Daywalker that it has already understood human behavior because it is also half-human. The Daywalker relents and spreads its wings, conceding that Mina may be right before flying away. Mina then goes to wake Ciara up.
Mina then reunites with her twin sister Lucy after missing their mother’s memorial. Mina recounts her experience to Lucy, saying that she still feels like she sees “Madeline” appearing to her in the forms of other people. Lucy’s sons then come to show Mina a drawing of her and Darwin, who she still keeps as a pet. Outside, the Daywalker appears staring at the ladies in the apartment, taking the form of a girl with a not fully human face.
